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From Andrey Gura <ag...@apache.org>
Subject Re: Consistency check and fix (review request)
Date Mon, 15 Apr 2019 12:58:21 GMT
Anton,

what does expression "withConsistency" mean? From user's standpoint it
means that all operations performed without this proxy are not
consistent. It means also that at least method name is bad.

Are there any guarantees that withConsistency proxy will not contain
bugs that will lead to inconsistent write after inconsistency was
found? I think there are no such guarantees. Bugs still are possible.
So I always must use withConsistency proxy because I doesn't have
other choice - all ways are unreliable and withConsistency just sounds
better.

Eventually we will have two different ways for working with cache
values with different bugs set. What is the profit?



On Fri, Apr 12, 2019 at 2:49 PM Anton Vinogradov <av@apache.org> wrote:
>
> Folks,
>
> I've checked the tx benchmarks and found no performance drop.
> Also, see no issues at TC results.
> So, seems, code ready to be merged.
>
> Everyone interested, please share any objections about
> - public API
> - test coverage
> - implementation approach
>
> On Wed, Apr 3, 2019 at 5:46 PM Anton Vinogradov <av@apache.org> wrote:
>
> > Nikolay,
> >
> > This is not a PoC, but the final solution (I hope so:) ) required the
> > review.
> > LWW means Last Write Wins, detailed explanation can be found at IEP-31.
> >
> > On Wed, Apr 3, 2019 at 5:24 PM Nikolay Izhikov <nizhikov@apache.org>
> > wrote:
> >
> >> Hello, Anton.
> >>
> >> Thanks for the PoC.
> >>
> >> > finds correct values according to LWW strategy
> >>
> >> Can you, please, clarify what is LWW strategy?
> >>
> >> В Ср, 03/04/2019 в 17:19 +0300, Anton Vinogradov пишет:
> >> > Ilya,
> >> >
> >> > This is impossible due to a conflict between some isolation levels and
> >> > get-with-consistency expectations.
> >> > Basically, it's impossible to perform get-with-consistency after the
> >> other
> >> > get at !READ_COMMITTED transaction.
> >> > The problem here is that value should be cached according to the
> >> isolation
> >> > level, so get-with-consistency is restricted in this case.
> >> > Same problem we have at case get-with-consistency after put, so we have
> >> > restriction here too.
> >> > So, the order matter. :)
> >> >
> >> > See OperationRestrictionsCacheConsistencyTest [1] for details.
> >> >
> >> > [1]
> >> >
> >> https://github.com/apache/ignite/blob/8b0b0c3e1bde93ff9c4eb5667d794dd64a8b06f0/modules/core/src/test/java/org/apache/ignite/internal/processors/cache/consistency/OperationRestrictionsCacheConsistencyTest.java
> >> >
> >> > On Wed, Apr 3, 2019 at 4:54 PM Ilya Kasnacheev <
> >> ilya.kasnacheev@gmail.com>
> >> > wrote:
> >> >
> >> > > Hello!
> >> > >
> >> > > Sounds useful especially for new feature development.
> >> > >
> >> > > Can you do a run of all tests with cache.forConsistency(), see if
> >> there are
> >> > > cases that fail?
> >> > >
> >> > > Regards,
> >> > > --
> >> > > Ilya Kasnacheev
> >> > >
> >> > >
> >> > > ср, 3 апр. 2019 г. в 16:17, Anton Vinogradov <av@apache.org>:
> >> > >
> >> > > > Igniters,
> >> > > >
> >> > > > Sometimes, at real deployment, we're faced with inconsistent
state
> >> across
> >> > > > the topology.
> >> > > > This means that somehow we have different values for the same
key at
> >> > > > different nodes.
> >> > > > This is an extremely rare situation, but, when you have thousands
of
> >> > > > terabytes of data, this can be a real problem.
> >> > > >
> >> > > > Apache Ignite provides a consistency guarantee, each affinity
node
> >> should
> >> > > > contain the same value for the same key, at least eventually.
> >> > > > But this guarantee can be violated because of bugs, see IEP-31
[1]
> >> for
> >> > > > details.
> >> > > >
> >> > > > So, I created the issue [2] to handle such situations.
> >> > > > The main idea is to have a special cache.withConsistency() proxy
> >> allows
> >> > > > checking a fix inconsistency on get operation.
> >> > > >
> >> > > > I've created PR [3] with following improvements (when
> >> > > > cache.withConsistency() proxy used):
> >> > > >
> >> > > > - PESSIMISTIC && !READ_COMMITTED transaction
> >> > > > -- checks values across the topology (under locks),
> >> > > > -- finds correct values according to LWW strategy,
> >> > > > -- records special event in case consistency violation found
> >> (contains
> >> > > > inconsistent map <Node, <K,V>> and last values <K,V>),
> >> > > > -- enlists writes with latest value for each inconsistent key,
so
> >> it will
> >> > > > be written on tx.commit().
> >> > > >
> >> > > > - OPTIMISTIC || READ_COMMITTED transactions
> >> > > > -- checks values across the topology (not under locks, so
> >> false-positive
> >> > > > case is possible),
> >> > > > -- starts PESSIMISTIC && SERIALIZABLE (at separate thread)
> >> transaction
> >> > >
> >> > > for
> >> > > > each possibly broken key and fixes it on a commit if necessary.
> >> > > > -- original transaction performs get-after-fix and can be continued
> >> if
> >> > >
> >> > > the
> >> > > > fix does not conflict with isolation level.
> >> > > >
> >> > > > Future plans
> >> > > > - Consistency guard (special process periodically checks we have
no
> >> > > > inconsistency).
> >> > > > - MVCC support.
> >> > > > - Atomic caches support.
> >> > > > - Thin client support.
> >> > > > - SQL support.
> >> > > > - Read-with-consistency before the write operation.
> >> > > > - Single key read-with-consistency optimization, now the collection
> >> > > > approach used each time.
> >> > > > - Do not perform read-with-consistency for the key in case it
was
> >> > > > consistently read some time ago.
> >> > > >
> >> > > > [1]
> >> > > >
> >> > > >
> >> > >
> >> > >
> >> https://cwiki.apache.org/confluence/display/IGNITE/IEP-31+Consistency+check+and+fix
> >> > > > [2] https://issues.apache.org/jira/browse/IGNITE-10663
> >> > > > [3] https://github.com/apache/ignite/pull/5656
> >> > > >
> >>
> >

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